The photorefractive effect, based on charge transport in electro-optic materials, is an optical nonlinearity that is useful for optical image processing. When a non-uniform pattern of illumination at an appropriate wavelength impinges on a photorefractive medium, the illumination pattern is temporarily stored in the medium as a pattern of refractive index variations. In typical applications, the pattern, which may, for example, be a diffraction grating or a hologram, is capable of diffracting a beam of light, here called the operand beam, that is propagated through it. As a result of diffraction by the medium, the operand beam upon exiting the medium is in a different state than when it entered. If the stored pattern is for the purpose of, e.g., modulation, the change of state represents the imposition of a signal onto the operand beam. Alternatively, two or more beams simultaneously present in the medium may interact in a more complex way such as to produce, e.g., the cross correlation of two spatially varying intensity patterns, or a holographic interferogram. (See P. Gunter and J. P. Huignard, The Photorefractive Effect, Springer Verlag, 1987, for reviews of applications.)
The photorefractive effect is based on the photoexcitation of free electric charge carriers (electrons or holes or both) in electro-optic crystals. The carriers then migrate away from the illuminated regions to trappidg sites, either by diffusion or by drift in an externally applied electric field. The resulting redistribution of charge creates space-charge fields within the crystal, which in turn create refractive index variations via the electro-optic effect. The refractive index change per absorbed photon in electro-optic materials is proportional to n.sub.i.sup.3 r.sub.ij /.epsilon..sub.j, where n.sub.i is the refractive index, r.sub.ij is the linear electro-optic coefficient, and .epsilon..sub.j is the dielectric constant. Some typical materials exhibiting relatively large values of this figure of merit are BaTiO.sub.3, Bi.sub.12 SiO.sub.20, Sr.sub.1-x Ba.sub.x Nb.sub.2 O.sub.5, InP, GaAs, and CdTe. In all of these materials, when optimized with the appropriate absorbing centers and trapping centers, the energy required per bit for photorefractive image processing is of the order of 1 pJ for a one square micrometer bit. The relaxation time depends only on the dielectric relaxation time of the material, and is therefore generally longer for insulators than for semiconductors. Over the range of materials of interest, the relaxation time varies typically from nanoseconds to seconds, although materials may exhibit both faster or slower relaxation times. When the relaxation is relatively slow, laser intensities less than 1 mW/cm.sup.2 are often sufficient to perform real-time image processing at diffraction-limited spatial resolution. The theory and applications of optical materials are discussed, for example, in A. M. Glass, "Materials for Optical Information Processing," Science, Vol. 226 (1984) pp. 657-662.
Another non-linear optical effect, analogous to the linear electro-optic effect, is the resonant electroabsorption that occurs near the band edge of layered semiconductor materials in the form of multiple quantum wells (MQWs), such as GaAs-AlGaAs multilayer heterostructures. The quantum well bound states of these structures give rise to a sharp room-temperature excitonic absorption peak in the heterostructure absorption spectrum near the band edge. The shape of the exciton peak, and therefore the optical absorptive and refractive effects associated with it, is sensitive to internal electric fields within the material. (The sensitivity of the optical absorption coefficient to electric fields is called "electroabsorption.") When an electric field is applied perpendicular to the quantum well layers, this effect is called the quantum confined Stark effect (QCSE), and when the electric field is applied parallel to the layers, the broadening is due to field-ionization. Internal electric fields, arising in response to applied external electric fields, change the electron and hole energy levels of the quantum wells. As a consequence, the exciton peak is broadened and/or shifted to lower energy. Because this effect is a resonant effect (that is, the incident light is at a wavelength close to the exciton absorption), it is generally much more sensitive to electric fields than the non-resonant linear electro-optic effect typically used in bulk photorefractive crystals.
One device that makes use of the electroabsorption is the self-electro-optic-effect device (SEED). In a SEED, the heterostructure is biased through a series resistor. Photocurrent resulting from optical absorption in the quantum wells causes a voltage drop across the resistor, reducing the applied field in the heterostructure and, as a consequence, changing its optical absorption. SEEDs are advantageous for optical switching because only very thin layers, typically of about one micrometer thickness, are needed. By contrast, photorefractive bulk crystals require an optical interaction length of about one centimeter. Moreover, as a consequence of the high sensitivity of the electroabsorption, SEEDs require very small amounts of optical energy. For example, an on-off contrast ratio of 2:1 in transmission can be achieved using only about 4 fJ per square micrometer of optical energy. However, SEEDs suffer from the disadvantage that because the heterostructure conducts electricity in the directions parallel to the layers, parallel multichannel processing, e.g., image processing, is possible only if individual pixels are reticulated and provided with individual electrical contacts. For example, the individual pixels are typically isolated by etching mesas using photolithographic techniques. This limits the spatial resolution obtainable to the resolution of the lithographic processing required to provide reticulation and individual contacts. In particular, it is unlikely that diffraction-limited resolution can be achieved using this technology. The theory and applications of electroabsorption are discussed, for example, in A. M. Glass, "Materials for Photonic Switching and Information Processing," MRS Bulletin, Vol. 13, No. 8 (1988) pp. 16-20 and D. A. B. Miller et al, "Electric Field Dependence of Optical Absorption Near the Bandgap of Quantum Well Structures," Phys. Rev., Vol. B32 (1985) pp. 1043-1060.
Thus, in many applications, it is advantageous to use the large resonant electro-optic effects near the exciton states of quantum well structures because very short optical interaction lengths, typically about one micrometer, are required, and because very small optical energy, typically several pJ per square micron, is required. (In practice, because of the large pixel size typically obtained by conventional lithography, the energy required is a few pJ per pixel.) However, practitioners in the art have thus far been unable to provide an electro-optic heterostructure that is capable of parallel, multichannel, image processing at diffraction-limited resolution.